Bug 1611536 - Add stdint.h include for fixed-width integer types r=Ehsan

musl libc does not define these types through the other headers
included by this file, which causes errors when building RemoteUtils.cpp
(which indirectly includes nsCharTraits.h):

	error: unknown type name 'int32_t'

Differential Revision: https://phabricator.services.mozilla.com/D61040

--HG--
extra : moz-landing-system : lando

xpcom/string/nsCharTraits.h

@@ -8,6 +8,7 @@
 #define nsCharTraits_h___
 
 #include <ctype.h>   // for |EOF|, |WEOF|
+#include <stdint.h>  // for |uint32_t|
 #include <string.h>  // for |memcpy|, et al
 #include "mozilla/MemoryChecking.h"